Alan Argento

High strain-rate behavior of natural fiber-reinforced polymer composites:

The high strain-rate constitutive behavior of polymer composites with various natural fibers is studied. Hemp, hemp/glass hybrid, cellulose, and wheat straw-reinforced polymeric composites have been manufactured, and a split-Hopkinson pressure bar apparatus has been designed to measure the dynamic stress–strain response of the materials. Using the apparatus, compressive stress–strain curves have been obtained that reveal the materials’ constitutive characteristics at strain rates between 600 and 2400 strain/s. Primary findings indicate that natural fibers in thermoset composites dissipate energy at lower levels of stress and higher strain than glass-reinforced composites. In the case of thermoplastic matrices, the effect on energy dissipation of natural fibers vs. conventional talc reinforcements is highly dependent on resin properties. Natural fibers in polypropylene homopolymer show improved reinforcement but have degraded energy dissipation compared to talc. Whereas in polypropylene copolymer, natural fibers result in improved energy dissipation compared to talc. These data are useful for proper design, analysis, and simulation of lightweight biocomposites.

Thermo-mechanical analysis of frictional heating in ultrasonic spot welding of aluminium plates

The thermo-mechanical analysis of an ultrasonic spot welding process was performed using developed finite element models. Temperature–time histories of aluminium specimens during welding were successfully obtained from the analysis and validated by physical tests. It was shown that the effect of frictional heating on the plastic deformation of aluminium during ultrasonic spot welding was significant and should be included in the analysis of the welding process in order to achieve good correlation with the physical phenomenon.

Bending Natural Frequencies of Circular CFRP Shafts with a Metal Core or Casing

It has recently been shown that some carbon fiber-reinforced composite shafts with a metal core or casing are superior to all carbon fiber reinforced plastics (CFRP) or metal shafts from the viewpoint of the natural frequencies. However, it has not been reported whether or not the benefit of the metal/CFRP hybrid shafts exists in the general cases of various fiber angles, length-to-diameter ratios, material properties, and thicknesses of metal layers. In this study, the effects of a steel core or casing on the bending natural frequencies of CFRP shafts are analytically investigated. In order to calculate the frequencies, the equations of motion are obtained for metal/CFRP hybrid shafts. Several beam theories are evaluated, and differences between these theories in the prediction of the frequencies are also presented. It is found that, in certain cases, an optimum thickness of steel exists to maximize the frequencies of the steel/CFRP shafts. Parametric studies are also carried out over wide ranges of design factors, and design guidelines for the hybrid shafts are finally distilled.

Constitutive behavior of ocular tissues over a range of strain rates.

The constitutive behavior of bovine scleral and corneal tissues is measured in tension and compression, at quasi-static and moderate strain rates. Experiments are conducted at strain rates up to about 50 strain per second by a pneumatic testing system developed to overcome noise and measurement difficulties associated with the time dependent test of low impedance materials. Results for the tissues at room and the natural bovine body temperatures are similar and indicate that ocular tissue exhibits nonlinear stiffening for increasing strain rates, a phenomena termed rate hardening. For example, at a tensile strain rate of 29/s, corneal tissue is found to develop 10 times the stress that it does quasi-statically at the same strain. Thus, conventional constitutive models will grossly underpredict stresses occurring in the corneo-scleral shell due to moderate dynamic events. This has implication to the accuracy of ocular injury models, the study of the stress field in the corneo-scleral shell for glaucoma research and tonometry measurements. The measured data at various strain rates is represented using the general framework of a constitutive model that has been used to represent biological tissue mechanical data. Here it is extended to represent the measured data of the ocular tissues over the range of tested strain rates. Its form allows for straightforward incorporation in various numerical codes. The experimental and analytical methods developed here are felt to be applicable to the test of human ocular tissue.

Shear Behavior of Bovine Scleral Tissue

Ocular tissue properties have been widely studied in tension and compression for humans and a variety of animals. However, direct shear testing of the tissues of the sclera appear to be absent from the literature even though modeling, analyses, and anatomical studies have indicated that shear may play a role in the etiology of primary open angle glaucoma (POAG). In this work, the mechanical behavior of bovine scleral tissue in shear has been studied in both out-of-plane and in-plane modes of deformation. Stress-strain and relaxation tests were conducted on tissue specimens at controlled temperature and hydration focusing on trends related to specimen location and orientation. There was generally found to be no significant effect of specimen orientation and angular location in the globe on shear stiffness in both modes. The in-plane response, which is the primary load carrying mode, was found to be substantially stiffer than the out-of-plane mode. Also, within the in-plane studies, tissue further from the optic nerve was stiffer than the near tissue. The viscosity coefficient of the tissue varied insignificantly with distance from the optic nerve, but overall was much higher in-plane than out-of-plane.

Rotating Tapered Composite Shafts: Forced Torsional and Extensional Motions and Static Strength

Forced torsional and extensional motions and static strength of rotating, tapered, filament-wound composite shafts subject to deflection-dependent cutting forces typical of end-milling and boring operations are studied. The general Galerkin method is used to satisfy spatial dependence in the equations of motion and direct numerical integration is used to determine the systems response. The maximum stress criterion is used to investigate static failure.

Imaging of Scleral Collagen Deformation Using Combined Confocal Raman Microspectroscopy and Polarized Light Microscopy Techniques.

This work presents an optospectroscopic characterization technique for soft tissue microstructure using site-matched confocal Raman microspectroscopy and polarized light microscopy. Using the technique, the microstructure of soft tissue samples is directly observed by polarized light microscopy during loading while spatially correlated spectroscopic information is extracted from the same plane, verifying the orientation and arrangement of the collagen fibers. Results show the response and orientation of the collagen fiber arrangement in its native state as well as during tensile and compressive loadings in a porcine sclera model. An example is also given showing how the data can be used with a finite element program to estimate the strain in individual collagen fibers. The measurements demonstrate features that indicate microstructural reorganization and damage of the sclera’s collagen fiber arrangement under loading. The site-matched confocal Raman microspectroscopic characterization of the tissue provides a qualitative measure to relate the change in fibrillar arrangement with possible chemical damage to the collagen microstructure. Tests and analyses presented here can potentially be used to determine the stress-strain behavior, and fiber reorganization of the collagen microstructure in soft tissue during viscoelastic response.

Perilimbal sclera mechanical properties: Impact on intraocular pressure in porcine eyes

There is extensive knowledge on the relationship of posterior scleral biomechanics and intraocular pressure (IOP) load on glaucomatous optic neuropathy; however, the role for biomechanical influence of the perilimbal scleral tissue on the aqueous humor drainage pathway, including the distal venous outflow system, and IOP regulation is not fully understood. The purpose of this work is to study the outflow characteristics of perfused porcine eyes relative to the biomechanical properties of the perilimbal sclera, the posterior sclera and the cornea. Enucleated porcine eyes from eleven different animals were perfused with surrogate aqueous at two fixed flow rates while monitoring their IOP. After perfusion, mechanical stress-strain and relaxation tests were conducted on specimens of perilimbal sclera, posterior sclera, and cornea from the same perfused eyes. Statistical analysis of the data demonstrated a strong correlation between increased tangent modulus of the perilimbal sclera tissues and increased perfusion IOP (R2 = 0.74, p = 0.0006 at lower flow rate and R2 = 0.71, p = 0.0011 at higher flow rate). In contrast, there were no significant correlations between IOP and the tangent modulus of the other tissues (Posterior sclera: R2 = 0.17 at lower flow rate and R2 = 0.30 at higher flow rate; cornea: R2 = 0.02 at lower flow rate and R2<0.01 at higher flow rate) nor the viscoelastic properties of any tissue (R2 ≤ 0.08 in all cases). Additionally, the correlation occurred for IOP and not net outflow facility (R2 ≤ 0.12 in all cases). These results provide new evidence that IOP in perfused porcine eyes is strongly influenced by the tangent modulus, sometimes called the tissue stiffness, of the most anterior portion of the sclera, i.e. the limbus.

Spray Deposition of Metals over Circular CFRP Core Shafts

It was theoretically shown by the authors in a previous article that by adding a layer of metal on the outside of a carbon fiber-reinforced composite shaft, the bending natural frequencies of the shaft could be increased in some cases due to reduction of the effects of shear deformation by the metal layer. In the present study, composite shafts having a metal casing have been manufactured by spray deposition process over filament wound, fiber reinforced polymeric shafts. The natural frequencies of the spray deposited composite shafts have been measured and found to compare well to theoretically calculated values. The experiments also confirm the shear deformation effect described above. Some manufacturing issues on the spray deposition of metals over carbon fiber-reinforced plastics are addressed.